Design Guide. Performance And Value With Engineering Plastics

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1 Design Guide Performance And Value With Engineering Plastics

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3 Table of Contents Overview of Design Principles 2 Dimensional Stability 4 Shrinkage 6 Design Guidelines 8 Wall Thickness 8 Warpage 10 Ribs and Profiled Structures 12 Gussets or Support Ribs 15 Bosses 15 Holes 17 Radii & Corners 19 Tolerances 20 Coring 20 Undercuts 21 Draft Angle 22 Mold Design Guidelines 23 Mold Machine 23 Mold Construction 24 Multi-Cavity Molds 25 Cold Runner Systems 26 Hot Manifold / Runnerless Molds 29 Gate Design 32 Mold Cooling 40 Ejection System 45 Type of Tool Steel 46 Surface Finish 47 Venting 48 1

4 Overview of Design Principles Figure 1 Molecular structure of thermoplastics. The process of developing thermoplastic parts requires a full understanding of typical material properties under various conditions. Thermoplastics can be categorized by their molecular structure as either amorphous, semi-crystalline plastics, or liquid crystal polymers (LCPs). The microstructures of these plastics and the effects of heating and cooling on the microstructures are shown in Figure 1. Amorphous thermoplastics. Amorphous polymers have a structure that shows no regularity. In an unstressed molten state polymer molecules are randomly oriented and entangled with other molecules. Amorphous materials retain this type of entangled and disordered molecular configuration regardless of their states. Only after heat treatment some small degree of orientation can be observed (physical aging). When the temperature of the melt decreases, amorphous polymers start becoming rubbery. When the temperature is further reduced to below the glass transition temperature, the amorphous polymers turn into glassy materials. Amorphous polymers possess a wide softening range (with no distinct melting temperature), moderate heat resistance, good impact resistance, and low shrinkage. Figure 2 Amorphous versus semi-crystalline thermoplastics. 2

5 Semi-crystalline thermoplastics. Semi-crystalline plastics, in their solid state, show local regular crystalline structures dispersed in an amorphous phase. These crystalline structures are formed when semi-crystalline plastics cool down from melt to solid state. The polymer chains are partly able to create a compacted structure with a relatively high density. The degree of crystallization depends on the length and the mobility of the polymer segments, the use of nucleants, the melt, and the mold temperatures. Figure 3 Glass transition temperature (Tg) and melt temperature (Tm). Liquid crystal polymers. Liquid crystal polymers (LCPs) exhibit ordered molecular arrangements in both the melt and solid states. Their stiff, rod-like molecules that form the parallel arrays or domains characterize these materials. The difference in molecular structure may cause remarkable differences in properties. Various properties are time or temperature dependent. The shear modulus, for instance, decreases at elevated temperatures. The shear modulus curve illustrates the temperature limits of a thermoplastic. The shape of the curve is different for amorphous and semicrystalline thermoplastics (see Figure 3) Figure 4 The following graph demonstrates time dependent creep moduli. In general semi-crystalline materials have lower creep rates than amorphous materials. Glass reinforcement generally improves the creep resistance of a thermoplastic material. 3

6 Figure 5 Shrinkage indication of DSM polymers in %. Dimensional Stability The following information on mold shrinkage, thermal expansion, and water absorption relates to the precision of a component both during molding and as secondary effects after molding. Mold shrinkage. Shrinkage or mold shrinkage is the difference between the mold cavity dimensions and the corresponding component dimensions. It is not possible to predict exact shrinkage values for a specific polymer grade. Therefore, the maximum and minimum values for the various DSM thermoplastics are provided in Figure 5. Figure 6 Dimensional stability through time %. During injection molding the polymer melt is injected into the mold. Once the mold is completely filled the dimensions of the molding are the same as the dimensions of the mold cavity at its service temperature (see "B" in Figure 6). While cooling down, the polymer starts to shrink (see "C" in Figure 6). During the holding stage of the injection molding cycle, shrinkage is compensated by postfilling/packing. Both the design of the part as well as the runner/gate should allow for sufficient filling and packing. The process of shrinkage continues even after the part has been ejected. Shrinkage should be measured long enough after injection molding to take into account post-shrinkage (see "D" in Figure 6). Secondary effects. If components are heated after molding, for example during paint curing operation, this can cause temporary or even permanent dimensional changes. The operating environment will also have consequences for the dimensional stability of the component. Thermal expansion. An important condition for the dimensions of a part is the use temperature. Thermoplastics show a relatively high thermal expansion (10-4 / C) compared to metals (10-5 / C). Thermal expansion cannot be ignored for large parts that are used at elevated temperatures (see "F" in Figure 6). 4

7 Moisture absorption. Akulon and Stanyl parts, like all polyamide moldings, show dimensional changes increase after molding due to moisture absorption (see Figure 7). Moisture absorption is a time dependent, reversible process that continues until equilibrium is reached. This equilibrium depends on temperature, relative humidity of the environment and the wall thickness of the molding. Figure 7 Effect of time and humidity on moisture absorption. A change in moisture content will result in different product dimensions. The designer should anticipate varying humidity conditions during use of the product (see E in Figure 6). The moisture absorption of reinforced grades differs from those of the unfilled grades. The moisture content not only affects the dimensions but also various important properties. Yield stress, modulus of elasticity and hardness decrease with increasing moisture absorption, while toughness shows a considerable increase. Figure 8 Dimensional stability of Akulon PA6/PA66 and Stanyl PA46. Although polyamide moldings are already comparatively tough in the dry state, the high toughness, which is characteristic of Akulon and Stanyl, is not reached until the material has absorbed moisture. Unreinforced Stanyl already shows a dry as molded impact resistance twice as high as other polyamides, so conditioning is less critical. Example of dimensional stability. Examples of the dimensional stability of unfilled and reinforced Akulon and Stanyl are shown in Figure 8. For polyamide grades in general, the swelling of the thickness is substantial, especially when compared to the swelling in the two other directions. This should be taken into account when designing parts with thick walls. Dimensional deviations/tolerances. All factors discussed influence the final dimensions of the part. The maximum dimensional deviation of the part is the sum of the individual contributing factors (see Figure 6). 5

8 Figure 9 pvt curves for amorphous and crystalline polymers. Shrinkage Shrinkage is inherent in the injection molding process. Shrinkage occurs because the density of polymer varies from the processing temperature to the ambient temperature. The shrinkage of ed plastic parts can be as much as 20 percent by volume, when measured at the processing temperature and the ambient temperature. Figure 10 Processing and design parameters that affect part shrinkage. Semi-crystalline materials are particularly prone to thermal shrinkage; amorphous materials tend to shrink less. When crystalline materials are cooled below their transition temperature, the molecules arrange themselves in a more orderly way, forming crystallites. On the other hand, the microstructure of amorphous materials does not change with the phase change. This difference leads to semi-crystalline materials having a greater difference in specific volume between the processing state (point A) and the state at room temperature and atmospheric pressure (point B) than amorphous materials (seen Figure 9). During injection molding, the variation in shrinkage both globally and through the cross section of a part creates internal stresses, (residual stresses). If the residual stresses are high enough to overcome the structural integrity of the part, the part will warp upon ejection from the mold or crack with external service load. Uncompensated volumetric contraction leads to either sink marks or voids in the interior of the part. Shrinkage that leads to sink marks or voids can be reduced or eliminated by packing the cavity after filling. Controlling part shrinkage is particularly important in applications requiring tight tolerances. 6

9 Excessive shrinkage can be caused by a number of factors: Figure 11 Relation between the shrinkage of glass fiber reinforced plastics and the orientation of the glass fibers (in thickness direction). - Low effective holding pressure - Short pack-hold time or cooling time - Fast freezing off of gate - High melt temperature - High temperature The relationship of shrinkage to several processing parameters and part thickness is shown schematically in Figure 10. Isotropic versus anisotropic shrinkage. For both unfilled amorphous and mineral-reinforced thermoplastics shrinkage is largely isotropic; shrinkage in flow direction is about equal to the shrinkage across flow. The glass fiber reinforced grades, on the other hand, show anisotropic properties. Due to fiber orientation in the direction of the melt flow, shrinkage values in flow direction often are substantially smaller than across flow direction (see Figure 11). The assumption a material has isotropic properties is often a good starting point but if anisotopy is totally ignored significant errors can occur in the design of thermoplastic parts. The designer must be aware that as the degree of anistopy increases so does the number of moduli required to describe the material, up to a maximum of 21. Therefore especially for glass-reinforced materials the use of simple analysis techniques has limited value and extensive FEA (finite element analysis) methods are often required to analyze anistropic materials in critical applications. Not only fiber orientation but also molecular orientation can lead to anistropic shrinkage. An unfilled molded part containing high levels of molecular orientation, due to high shear stresses, can show anistropic shrinkage because aligned chains shrink more in the direction of orientation. 7

10 Design Guidelines Figure 12 Spiral flow length of Akulon Ultraflow at 260 C and 1400 bar. Wall Thickness Just as metals have normal working thickness ranges based upon their processing method, so do plastics. Typically, for injection molded parts, the wall thickness will be in the range 0.5 mm to 4 mm ( in). Dependent on the part design and size, parts with either thinner or thicker sections can be molded. Figure 13 Gradual transition of wall thicknesses. While observing functional requirements, keep wall thicknesses as thin and uniform as possible. In this way even filling of the mold and anticipated shrinkage throughout the molding can be obtained in the best way. Internal stresses can be reduced. Wall thickness should be minimized to shorten the molding cycle, obtain low part weight, and optimize material usage. The minimum wall thickness that can be used in injection molding depends on the structural requirements, the size and geometry of the molding, and the flow behavior of the material. As a starting point the designer can often refer to spiral flow curves which give a relative measure of the maximum achievable flow length for a given wall thickness and injection pressure. See Figure 12. If parts are subjected to any significant loading the part should be analyzed for stress and deflection. If the calculated stress or deflection value is not acceptable a number of options could be considered including the following: Increase wall (if not already too thick) Use an alternative material with higher strength and/or modulus Incorporate ribs or contours in the design to increase the sectional modulus 8

11 Other aspects that may need to be considered include: Insulation characteristics Generally speaking insulating ability (whether for electrical or heat energy) is related to the thickness of the polymer. Figure 14 Sink marks due to large wall thicknesses. Figure 15 Voids due to large wall thicknesses. Impact characteristics Impact resistance is directly related to the ability of a part to absorb mechanical energy without fracturing. This in turn is related to the part design and polymer properties. Increasing the wall section will generally help with impact resistance but too thick (stiff) a section may make a design unable to deflect and distribute an impact load therefore increasing stresses to an unacceptable level. Figure 16 Influence of rib design on flow behavior of the melt. Agency approval When a part design must meet agency requirements for flammability, heat resistance, electrical properties etc, it may be necessary to design with thicker sections than would be required just to meet the mechanical requirements. Where varying wall thicknesses are unavoidable for reasons of design, there should be a gradual transition (3 to 1) as indicated in Figure 13. Generally, the maximum wall thickness used should not exceed 4 mm (0.16 in). Thicker walls increase material consumption, lengthen cycle time considerably, and cause high internal stresses, sink marks and voids (see Figure 14). Figure 17 Example of design study of multi-connector. Care should be applied to avoid a "race tracking" effect, which occurs because melt preferentially flows faster along thick sections. This could result in air traps and welds lines, which would appear as surface defects. Modifying or incorporating ribs in the design can often improve thick sections. 9

12 Figure 18 Differential shrinkage for filled and unfilled materials. Warpage If the shrinkage throughout the part is uniform, the molding will not deform or warp; it simply becomes smaller. Warpage can be considered as a distortion where the surfaces of the molded part do not follow the intended shape of the design. Figure 19 Unreinforced vs fiber reinforced materials. Figure 20 Part warpage due to: (a) non-uniform cooling in the part and (b) asymmetric cooling across the part thickness. Part warpage results from differential shrinkage in the material of the molded part caused by molded-in residual stresses. Variations in shrinkage can be caused by molecular and fiber orientation, temperature variations within the molded part, and by variable packing, such as over-packing at gates and under-packing at remote locations, or different pressure levels as material solidify across the part thickness. Due to the number of factors present it is a very complicated task to achieve uniform shrinkage. These causes are described more fully in the information that follows. Influence of unfilled and filled materials. For fiber reinforced thermoplastics, reinforcing fibers inhibit shrinkage due to their smaller thermal contraction and higher modulus. Therefore, fiber reinforced materials shrink less along the direction in which fibers align (typically the flow direction) compared to the shrinkage in the transverse direction. Similarly, particle-filled thermoplastics shrink less than unfilled grades, but exhibit a more isotropic nature. For non-reinforced materials warpage is generally influenced by wall thickness and mold temperature. If wall thickness and mold temperatures are not optimal the molding will most likely warp. For glass reinforced materials totally different characteristics are evident due to fiber orientation. If a non-reinforced and a fiber reinforced material are compared in the same design it is possible to see contrary warpage in the same part. 10

13 Influence of cooling. Non-uniform cooling in the part and asymmetric cooling across the part thickness from the cavity and core can also induce differential shrinkage. The material cools and shrinks inconsistently from the wall to the center, causing warpage after ejection. Influence of wall thickness. Shrinkage increases as the wall thickness increases. Differential shrinkage due to non-uniform wall thickness is a major cause of part warpage in unreinforced thermoplastics. More specifically, different cooling rates and crystallization levels generally arise within parts with wall sections of varying thickness. Larger volumetric shrinkage due to the high crystallization level in the slow cooling areas leads to differential shrinkage and thus part warpage. Figure 21 Diagram of high shrinkage/low cooling vs warped part. Figure 22 Poor design vs warped part. Influence of asymmetric geometry. Geometric asymmetry (e.g., a flat plate with a large number of ribs that are aligned in one direction or on one side of the part) will introduce non-uniform cooling and differential shrinkage that can lead to part warpage. The poor cooling of the wall on the ribbed side causes a slower cooling of the material on that one side, which can lead to part warpage. 11

14 Figure 23 Recommendations for rib dimensions. Ribs and Profiles Structures If the load carrying ability or the stiffness of a plastic structure needs to be improved it is necessary to either increase the sectional properties of the structure or change the material. Changing the material or grade of material, e.g. higher glass fiber content, may be adequate sometimes but is often not practical (different shrinkage value) or economical. Increasing the sectional properties, namely the moment of inertia, is often the preferred option. As discussed in other sections, just increasing the wall section although the most practical option will be self-defeating. Figure 24 Ribs. Increase in part weight and costs are proportional to the increase in thickness. Increase in cooling time is proportional to the square of the increase in thickness. If the load on a structural part requires sections exceeding 4 mm (0.16 in) thickness, reinforcement by means of ribs or box sections is advisable in order to obtain the required strength at an acceptable wall thickness. Figure 25 Corrugations. Solid plate vs. ribbed plate in terms of weight and stiffness. Although ribs offer structural advantages they can give rise to warpage and appearance problems, for this reason certain guidelines should be followed: The thickness of a rib should not exceed half the thickness of the nominal wall as indicated in Figure 23. In areas where structure is more important than appearance, or with very low shrinkage materials, ribs with a thickness larger than half the wall thickness can be used. These will cause sink marks on the surface of the wall opposite the ribs. In addition, thick ribs may act as flow leaders causing preferential flows during injection. This results in weld lines and air entrapment. 12

15 Maximum rib height should not exceed 3 times the nominal wall thickness as deep ribs become difficult to fill and may stick in the mold during ejection. Typical draft is 1 to 1.5 deg per side with a minimum of 0.5 deg per side. Generally draft and thickness requirements will limit the rib height. At the intersection of the rib base and the nominal wall a radius of 25 to 50% of the nominal wall section should be included. Minimum value 0.4 mm. This radius will eliminate a potential stress concentration and improve flow and cooling characteristics around the rib. Larger radii will give only marginal improvement and increase the risk of sink marks on the opposite side of the wall. Parallel ribs should be spaced at a minimum distance of twice the nominal wall thickness; this helps prevent cooling problems and the use thin blades in the mold construction. Figure 26 Flat and open areas. Figure 27 Dimension image with chart of case 1-6. Ribs are preferably designed parallel to the melt flow as flow across ribs can result in a branched flow leading to trapped gas or hesitation. Hesitation can increase internal stresses and short shots. Parallel ribs should be spaced at a minimum distance of twice the nominal wall thickness; this helps prevent cooling problems and the use thin blades in the mold construction. Ribs are preferably designed parallel to the melt flow as flow across ribs can result in a branched flow leading to trapped gas or hesitation. Hesitation can increase internal stresses and short shots. Ribs should be orientated along the axis of bending in order to provide maximum stiffness. Consider the example in Figure 24 where a long thin plate is simply supported at the ends. If ribs are added in the length direction the plate is significantly stiffened. However, if ribs are added across the width of the plate little improvement is found. 13

16 Figure 28 Comparison of profiles in terms of torsional rigidity and bending. Ribbing is typically applied for: Increasing bending stiffness or strength of large flat areas Increasing torsional stiffness of open sections Adding corrugations to the design can stiffen flat surfaces in the direction of the corrugations (see Figure 25). They are very efficient and do not add large amounts of extra material or lengthen the cooling time. The extra stiffness is a result of increasing the average distance of the material from the neutral axis of the part, i.e. increasing the second moment of inertia. Ribs and box sections increase stiffness, thus improving the load bearing capability of the molding. These reinforcing methods permit a decrease in wall thickness but impart the same strength to the section as a greater wall thickness. Figure 29 Torsional rigidity and resistance to torsional stress as a function of the way in which the ribs are connected to the profile. The results demonstrate that the use of diagonal ribs have the greatest effect on the torsional rigidity of the section. The change from an I section to a C section helps in terms of horizontal bending terms but not in torsional terms. As double cross ribs (Figure 28 - option 6) can give tooling (cooling) problems option 8 is the recommended solution for the best torsional performance. Depending on the requirements of the part the acceptability of possible sink marks at the intersection of the ribs and profile wall need special consideration. For maximum performance and function the neutral lines of the ribs and profile wall should meet at the same point. Deviation from this rule will result in a weaker geometry. If, due to aesthetic requirements, the diagonal ribs are moved slightly apart then the rigidity is reduced 35%. If a short vertical rib is added to the design then the torsional rigidity is reduced an additional 5% (see Figure 29). 14

17 Gussets or Support Ribs Figure 30 Guidelines for gussets. Gussets can be considered as a subset of ribs and the guidelines that apply to ribs are also valid for gussets. This type of support is used to reinforce corners, side walls, and bosses. The height of the gusset can be up to 95% of the height of the boss or rib it is attached to. Depending on the height of the rib being supported gussets may be more than 4 times the nominal wall thickness. Gusset base length is typically twice the nominal wall thickness. These values optimize the effectiveness of the gusset and the ease of molding and ejecting the part. Figure 31 Height of the gusset. Bosses Bosses often serve as mounting or fastening points and therefore, for good design, a compromise may have to be reached to achieve good appearance and adequate strength. Thick sections need to be avoided to minimize aesthetic problems such as sink marks. If the boss is to be used to accommodate self tapping screws or inserts the wall section must be controlled to avoid excessive build up of hoop stresses in the boss. General recommendations include the following: Nominal boss wall thickness less than 75% nominal wall thickness, note above 50% there is an increased risk of sink marks. Greater wall sections for increased strength will increase molded-in stresses and result in sink marks. A minimum radius of 25% the nominal wall thickness or 0.4 mm at the base of the boss is recommended to reduce stresses. A minimum draft of 0.5 degrees is required on the outside dimension of the boss to ensure release from the mold on ejection. 15

18 Figure 32 Proper boss design. Figure 33 Correct positioning of bosses. Increasing the length of the core pin so that it penetrates the nominal wall section can reduce the risk of sink marks. The core pin should be radiused (min 0.25 mm) to reduce material turbulence during filling and to help keep stresses to a minimum. This option does increase the risk of other surface defects on the opposite surface. A minimum draft of 0.25 degrees is required on the internal dimension for ejection and or proper engagement with a fastener. Further strength can be achieved with gusset ribs or by attaching the boss to a sidewall. Bosses adjacent to external walls should be positioned a minimum of 3 mm (.12 in) from the outside of the boss to avoid creating a material mass that could result in sink marks and extended cycle times (see Figure 33). A minimum distance of twice the nominal wall thickness should be used for determining the spacing between bosses (see Figure 34). If placed too close together thin areas that are hard to cool will be created. These will in turn affect quality and productivity. Figure 34 Boss spacing. 16

19 Holes Figure 35 Blind cores. Holes are easily produced in molded parts by core pins. Through holes are easier to produce than blind holes because the core pin can be supported at both ends. Blind holes. Core pins supported by just one side of the mold tool create blind holes. The length of the pins, and therefore the depth of the holes, are limited by the ability of the core pin to withstand any deflection imposed on it by the melt during the injection phase. See information on bosses & cores. As a general rule the depth of a blind hole should not exceed 3 times the diameter. For diameters less than 5 mm this ratio should be reduced to 2. Figure 36 Through cores. Through holes. With through holes the cores can be longer as the opposite side of the mold cavity can support them. An alternative is to use a split core fixed in both halves of the mold that interlock when the mold is closed. For through holes the length of a given size core can be twice that of a blind hole. In cases where even longer cores are required, careful tool design is necessary to ensure balanced pressure distribution on the core during filling to limit deflection. 17

20 Figure 37 Minimum hole spacing dimensions. Figure 38 Blind hole design recommendations. Holes with an axis that runs perpendicular to the mold opening direction require the use of retractable pins or split tools. In some designs placing steps or extreme taper in the wall can avoid this. See section on draft. Core pins should be draw polished and include draft to help with ejection. The mold design should direct the melt flow along the length of slots or depressions to locate weld lines in thicker or less critical sections. If weld lines are not permissible due to strength or appearance requirements, holes may be partially cored to facilitate drilling as a post molding operation. The distance between two holes or one hole and the parts edge should be at least 2 times the part thickness or 2 times the hole diameter whichever is the largest. For blind holes the thickness of the bottom should be greater than 20% of the hole diameter in order to eliminate surface defects on the opposite surface. A better design is to ensure the wall thickness remains uniform and there are no sharp corners where stress concentrations can occur. 18

21 Radii & Corner Figure 39 Corner radius. In the design of injection molded parts sharp corners should always be avoided; generous radii should be included in the design to reduce stress concentrations. Fillet radii should be between 25 and 60% of the nominal wall thickness. If the part has a load bearing function then the upper end is recommended. A minimum radius of 0.5 mm is suggested and all sharp corners should be broken with at least a mm radius. Sharp corners, particularly internal corners introduce: High molded in stresses Poor flow characteristics Reduced mechanical properties Increased tool wear Surface appearance problems, (especially with blends). Figure 40 Stress concentration as a function of wall thickness and corner radius. The inclusion of a radius will give: Uniform cooling Less warpage Less flow resistance Easier filling Lower stress concentration Less notch sensitivity. The outside corner radius should be equal to the inside radius plus the wall thickness as this will keep a uniform wall thickness and reduce stress concentrations. For a part with an internal radius half the nominal wall thickness a stress concentration factor of 1.5 is a reasonable assumption. For smaller radii, e.g. 10% of the nominal wall, this factor will increase to 3. Standard tables for stress concentration factors are available and should be consulted for critical applications. Figure 41 Sharp corners. In addition, from a molding view point, it is important is to avoid sharp internal corners. Due to the difference in area/volume-ratio of the polymer at the outside and the inside of the corner, the cooling at the outside is better than the cooling at the inside. As a result the material at the inside shows more shrinkage and so the corner tends to deflect (see Figure 41). 19

22 Figure 42 Characteristics of tolerance classes. Figure 43 Factors affecting parts tolerance. Tolerances Establishing the correct tolerances with respect to the product function is of economic importance. The designer should be aware that dimensions with tight tolerances have a big influence on the costs of both product and mold. Even slightly over specifying tolerances may adversely influence tool costs, injection molding conditions, and cycle time. It is recommended to indicate only critical dimensions with tolerances on a drawing. Depending on the application, a division into three tolerance classes can be made: normal; price index 100 accurate; technical injection molding; price index 170 precise; precision injection molding; price index 300 The most important characteristics of the tolerance classes are shown in Figure 42. Mold design, mold cavity dimensions, product shape, injection-molding conditions and material properties determine the tolerances that can be obtained. Figure 43 provides a summary of the factors that play a major role in establishing dimensional accuracy. Coring Coring refers to the elimination of plastic material in oversized dimensioned areas by adding steel to the mold tool that usually results in a pocket or opening in the part. For simplicity and economical reasons cores should ideally be placed parallel to the line of draw. Cores in other directions require the use of some form of side action (cam operated or hydraulic) to be actuated thus increasing tooling costs (see Figure 45). 20

23 Undercuts Undercuts should be avoided if possible through redesign of the part. Ideally, the mold tool should open in a direction parallel to the movement of the machine platen. In Figure 46 hanging the form of the hole reduces initial costs and also maintenance cost during production. For some complex parts the ideal situation will not exist and mechanical movement of one sort or another will be required. A description of possible movements includes the following: Figure 44 Coring designs. FIgure 45 Construction B could cost as much as 60% less than A. Deflection Dependent on the material and amount of undercut it may be possible to deflect the part out of the mold. Inserts The use of removable inserts that eject with the part is an option, certainly for prototype tooling. The disadvantages are the inserts must be removed from the ejected part and repositioned in the mold thus possibly extending the cycle time. Cams Cams or hydraulic/pneumatic cylinders move part of the mold out of the way to permit part ejection. These increase the complexity of the mold making it more expensive and also mean a controller is required to operate them during the molding cycle. Cycle times will also be affected. Figure 46 Undercuts. Slides By means of angled pins and rods mounted in the mold it may be possible to move the part of the mold forming the undercut in the direction of the angled pin during the opening sequence of the mold. This then allows ejection of the part. Stepped parting line By repositioning the parting line it may be possible to eliminate undercut features; although this may add to the complexity of the tool it is the most recommended solution. 21

24 Figure 47 Cammed mold for part with undercut cams move in vertical direction when mold is opened. Figure 47 shows an example of a sliding cam. The cam pins that operate the cams are mounted under a maximum angle of in the injection side. The angle is limited because of the enormous force that is exerted on these pins during mold opening and closing. Draft Angle Figure 48 Draft (A) in mm for various draft angles (B) as a function of molding depth (C). Figure 49 Parting line. Part features cut into the surface of the mold perpendicular to the parting line require taper or draft to permit proper ejection. This draft allows the part to break free by creating a clearance as soon as the mold starts to open. Since thermoplastics shrink as they cool they grip to cores or male forms in the mold making normal ejection difficult if draft is not included in the design. If careful consideration is given to the amount of draft and shutoff in the mold it is often possible to eliminate side actions and save on tool and maintenance costs. For untextured surfaces generally a minimum of 0.5 deg draft per side is recommended although there are exceptions when less may be acceptable. Polishing in draw line or using special surface treatments can help achieve this. For textured sidewalls use an additional 0.4 deg draft per 0.1mm depth of texture. Typically 1 to 3 deg draft is recommended. As the draft increases ejection becomes easier but it increases the risk that some sections may become too heavy. Try to keep features in the parting line or plane. When a stepped parting line is required allow 7 deg for shutoff. 5 deg should be considered as a minimum. Drag at the shutoff will cause wear over time with the risk that flash will form during molding. More frequent maintenance will be required for this type of tooling if flash free parts are to be produced. 22

25 Mold Design Guidelines Mold design and construction requires special attention for optimal product quality and reliable molding. Figure 50 Mold mounting dimensions. Mold Machine The mold should be tuned to the injection molding equipment with respect to mold mounting, injection unit and clamping force. Relevant molding machine data can be found in Figure 50. The maximum shot weight of the injection unit is the amount of plastic that can be injected per shot. The weight of the molding should not exceed 70% of the maximum shot weight. The minimum plasticizing capacity depends on the relationship between shot weight and cooling time. For example, molding 300 g in 30 seconds requires a minimum plasticating capacity of 36 kg/hr. The required clamping force of a molding machine is determined by the cavity pressure during the injection/ holding stage and the projected area of the part in the clamp direction. Various factors affect the molding pressure, e.g. length over thickness ratio of the molded part, injection speed and melt viscosity. Typical injection pressures are N/mm 2, resulting in a required clamp force of tons/cm 2. For thin wall moldings the required pressures can be a factor 2 higher resulting in a required clamp force of 1 tons/cm 2. With engineering structural foam molding pressures are significantly lower allowing for less robust mold design or the use of aluminum in place of steel in the mold construction. 23

26 Figure 51 Impression of a standard injection mold. Mold Construction A standard injection mold is made of a stationary or injection side containing one or more cavities and a moving or ejection side. Relevant details are shown in Figure 51. High quality molds are expensive because labor and numerous high- precision machining operations are time-consuming. Product development and manufacturing costs often can be significantly reduced if sufficient attention is paid to product and mold design. The way in which the mold is constructed is determined by: Figure 52 Cammed mold for part with undercut cams move in vertical direction when mold is opened. shape of the part number of cavities position and system of gating material viscosity mold venting A simple mold with a single parting line is shown in Figure 51. More complex molds for parts with undercuts or side cores may use several parting lines or sliding cores. These cores may be operated manually, mechanically, hydraulically, pneumatically or electromechanically. Figure 52 shows an example of a sliding cam. The cam pins that operate the cams are mounted under a maximum angle of in the injection side. The angle is limited because of the enormous force that is exerted on these pins during mold opening and closing. 24

27 Multi-Cavity Molds Figure 53 Total part costs in relation to number of cavities. The number of cavities and mold construction depend both on economical and technical factors. Important is the number of parts to be molded, the required time, and price in relation to mold manufacturing costs. Figure 53 shows the relation between the total part costs and the number of cavities. The gating system and gate location can limit the design freedom for multi-cavity molds. Dimensional accuracy and quality requirements should be accounted for. The runner layout of multiple-cavity molds should be designed for simultaneous and even cavity filling. The maximum number of cavities in a mold depends on the total cavity volume including runners in relation to the maximum barrel capacity and clamping force of the injection molding machine. Number of cavities. A given molding machine has a maximum barrel capacity of 254 cm 3, a plasticizing capacity of 25 g/s, 45 mm screw and a clamping force of 1300 kn. A PC part of 30 cm 3, (shot weight 36 g) and a projected area of 20 cm 2 including runners requires about 0.5/tons/cm 2 (5 kn/cm 2 ) clamping force. The maximum number of cavities based on the clamping force would be 12. It is advisable to use only 80% of the barrel capacity, thus the number of cavities in this example is limited to 6. When very short cycle times are expected the total number of cavities may be further reduced. A 6-cavity mold in this example requires a shot weight of 216 g. The cooling time must be at least 8.7 seconds. 25

28 Figure 54 Sprue design. Cold Runner Systems The runner system is a manifold for distribution of thermoplastic melt from the machine nozzle to the cavities. The sprue bushing and runners should be as short as possible to ensure limited pressure losses in the mold. Properly sizing a runner to a given part and mold design has a tremendous pay-off. To summarize a runner system that has been designed correctly will: Deliver melt to the cavities Balance filling of multiple cavities Balance filling of multi-gate cavities Minimize scrap Eject easily Maximize efficiency in energy consumption Control the filling/packing/cycle time. Figure 55 Cross sectional properties for various runner profiles. Sprue bushings. Sprue bushing connects the machine nozzle to the runner system. To ensure clean ejection from the bushing the bushing should have a smooth, tapered internal finish and have been polished in the direction of draw. The use of a positive sprue puller is recommended. A cold slug well should also be included in the design. This prevents a slug of cold material from entering the feed system and finally the part, which could affect the final properties of the finished part. The dimensions of the sprue depend primarily on the dimensions of the molded part in particular the wall thickness. As a guideline: The sprue must not freeze before any other cross section in order to permit sufficient transmission of holding pressure. The sprue must de- easily and reliably. Runner geometry. The ideal runner cross section is circular as this ensures favorable melt flow and cooling. However, it takes more effort to build circular runners because one half must be machined in the fixed mold part and the other half in the moving mold part. The higher the surface to volume ratio, the more efficient the runner. 26

29 Full trapezoidal channels in one of the two mold halves provide a cheaper alternative (see Figure 55). The rounded off trapezoidal cross section combines ease of machining in one mold half with a cross section that approaches the desired circular shape. The height of a trapezoidal runner must be at least 80% of the largest width. Half round runners are not recommended because of their low volume to surface ratio. Figure 56 Maximum runner length for specific diameters. Runner dimensions. The diameter of a runner highly depends on its length in addition to the part volume, part flow length, machine capacity, and gate size. Generally they must never be smaller than the largest wall thickness of the product and usually lie within the range 3 mm to 15 mm. Recommended runner dimensions are provided in Figure 56. The selection of a cold runner diameter should be based on standard machine tool cutter sizes Runner layout. There are 3 basic layout systems used for multi-cavity systems. These can be categorized as follows: Figure 57 Example of unbalanced feed systems. Standard (herringbone) runner system "H" bridge (branching) runner system Radial (star) runner system Unbalanced runner systems lead to unequal filling, post-filling and cooling of individual cavities that may cause failures like: Incomplete filling Differences in product properties Shrinkage differences/warpage Sink marks Flash Poor mold release Inconsistency Although the herringbone is naturally unbalanced, it can accommodate more cavities than its naturally balanced counterparts, with minimum runner volume and less tooling cost. With computer aided flow simulation it is possible to adjust primary and secondary runner dimensions to obtain equal filling patterns. 27

30 Figure 58 Naturally balanced feed systems. Keep in mind that non-standard runner diameters will increase manufacturing and maintenance costs. Adjusting runner dimensions to achieve equal filling may not be sufficient in critical parts to prevent potential failures. Special attention is required for: Very small components Parts with thin sections Parts that permit no sink marks Parts with a primary runner length much larger than secondary runner length. It is preferred to design naturally balanced runners as shown in Figure 58. The "H" (branching) and radial (star) systems are considered to be naturally balanced. The naturally balanced runner provides equal distance and runner size from the sprue to all the cavities, so that each cavity fills under the same conditions. When high quality and tight tolerances are required the cavities must be uniform. Figure 59 Recommended design of cold slug well or overflow. Family molds are not considered suitable. Nevertheless, it might be necessary for economical reasons to mold different parts in one mold. The cavity with the largest component should be placed nearest to the sprue. The maximum number of cavities in a mold depends on the total cavity volume including runners in relation to the maximum barrel capacity and clamping force of the injection molding machine. Branched runners. Each time a runner is branched, the diameter of the branch runners should be smaller than the main runner, because less material flows through the branches and it is economically desirable to use minimum material in the runners. Where N is the number of branches, the relationship between the main runner diameter (dmain) and the branch runner diameter (dbranch) is: dmain= dbranch x N1/3 At all runner intersections there should be a cold slug well. The cold slug well helps the flow of material through the runner system by stopping colder, higher viscosity material moving at the forefront of the molten mass entering into the cavity. 28

31 The length of the well is usually equal to or greater than the runner diameter and this is achieved by extending the length of the primary runner at the intersection with the secondary runner (see Figure 59). Figure 60 Insulated runner mold. While large runners facilitate the flow of material at relatively low pressure requirements, they require a longer cooling time, more material consumption and scrap, and more clamping force. Designing the smallest adequate runner system will maximize efficiency in both raw material use and energy consumption in molding. The runner size reduction is constrained by the molding machine's injection pressure capability. Initially runner diameters can be calculated with the following formula below. Further finetuning can then be performed with the use of flow analysis software where effects such as shear heating and skin layer formation can be taken into consideration. Hot Manifold / Runnerless Molds Runnerless molds differ from cold runner molds by extending the injection molding machine s melt chamber and acting as an extension of the machine nozzle. A portion or all of the polymer melt is at the same temperature and viscosity as the polymer in the barrel of the injection molding machine. There are two general types of runnerless molds the insulated system and the hot runner system. Insulated runner system. Insulated runner molds have oversized passages formed in the mold plate. The passages are of sufficient size that, under conditions of operation, the insulated effect of the plastic (frozen on the runner wall) combined with the heat applied with each shot maintains an open, molten flow path. Advantages of an insulated runner system compared to a conventional cold runner system include: Reduction in material shear Faster cycle times Elimination of runner scrap Decreased tool wear Improved part finish Less sensitivity to the requirements for balanced runners Shorter cycle times Disadvantages include: (Generally) more complex tool design (Generally) higher tool costs Higher maintenance costs Start up procedure is more difficult Possible thermal degradation of material More difficult to change color The insulated runner system should be designed so that while the runner volume does not exceed the cavity volume all of the molten material in the runner is injected into the cavity during each shot. This helps prevent excessive build-up of the insulating skin and minimizes any drop in melt temperature. 29

32 Figure 61 Cross section of a basic hot runner system. Hot runners. More commonly used than insulated runners are hot runners which fall into two categories: internally and externally heated. Hot runners retain the advantages of the insulated runner system over conventional cold runner system and eliminate a number of the disadvantages. The major disadvantages compared to a cold runner mold are: More complex mold design, manufacture operation and maintenance Substantially higher costs Thermal expansion of various components needs to be taken into account These disadvantages are a result of the need to install a heated manifold, balance heat generated by the manifold and the minimization of polymer hang-ups. Figure 61 shows a schematic cross-section of a hot runner system. It is often cost effective to produce large volumes with hot runner molds, in spite of high investments. These systems are used for a wide range of applications. The electrical/electronic industry uses small components, like connectors and bobbins that are molded in multi-cavity molds. On the other hand, large multi-gated parts are used in the automotive industry, e.g. bumpers and dashboards. Yet both can benefit from the cost and technical advantages of hot runners. Cycle time reduction is possible when cooling of a cold runner would determine the cycle time. Following are typical advantages and disadvantages of hot runner systems: Advantages Production increase (cycle) Material saving Quality improvement No waste Automatic degating Energy saving Flexible choice gate location 30

33 Disadvantages Higher investment Critical molding conditions Critical temperature control Start-up problems (tailing) Color change problems Abrasion (reinforced plastics) Critical mold design (no dead spots) Figure 62 Advantages and disadvantages of basic hot runner configurations. In selecting a hot runner system, the factors in Table 1 need to be taken into account. Taking all these factors into consideration, there is still a choice between many types and variations of hot runner manifolds and nozzles. General recommendations cannot be given. The best option depends on the thermoplastic and the requirements of the specific application. The following guidelines should be respected: Natural runner balancing Minimal pressure-losses Sufficient heating capacity for manifold and each single nozzle Accurate, separate temperature controls for manifold and nozzle Effective insulation between manifold and mold Optimal mold temperature control No dead spots and flow restrictions in manifold and nozzles Limited residence time of melt in the hot runner Adequate sealing of runners Figure 62 shows various basic types of nozzle configurations with their typical advantages and disadvantages. With respect to externally and internally heated manifolds the same conclusions are applicable as for nozzles. A relatively cheap and robust alternative for hot runners is the hot runner/ cold sprue. The hot runner manifold is followed by a short cold sprue that eliminates the use of expensive nozzles. Table 1 Factors to consider in selecting a hot runner system. Economy Product Investment Dimensions Number of parts Shot weight Cycle times Gate/sink marks Material waste Reproducibility Energy savings Required tolerances/warpage Regrinding Fiber orientation Process Material Start up Flow behavior Total flow path Melting temperature/range Pressure distribution Process window Melt homogeneity Thermal stability Color change Reinforcement Residence time Additives 31

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